Adsorption processes have been widely used for the separation and purification of gases. In recent years, pressure swing adsorption (PSA) systems including vacuum PSA (VPSA) have been developed for enhanced gas separation operations, particularly for the commercial production of oxygen and/or nitrogen from air. The PSA process consists of basic steps such as adsorption-desorption. Air or another gas mixture is fed into the system, which includes one or more vessels, each with an adsorbent bed, to an upper adsorption pressure. The adsorbent beds (a.k.a. adsorbers) selectively adsorb the more readily adsorbable component(s) of the air or gas mixture. The less readily adsorbable component(s) passes through the adsorber. The vessel is then depressurized to a lower desorption pressure for the desorption of the more readily adsorbable component(s) from the adsorber(s), which is then repressurized to the upper adsorption pressure as cyclic operation is continued.
In order to improve performance, conventional PSA systems typically use two or more vessels in parallel with directional valves to connect the vessels in alternating sequence to the compressor or vacuum pump. Further, to fully utilize the adsorbent material employed, PSA systems require uniform flow of gas across the adsorbent vessel(s) throughout the PSA processing cycle. In addition, void volume and pressure drops in the PSA vessel entrance and exit regions (i.e., the inlets and outlets and their associated headers) have adverse effects on the process performance of a PSA system and must be minimized in practical commercial operations. Ruthven, et al. disclose a method for estimating the magnitude of effects by void volume via an equilibrium model. More specifically, they examined the effects of void volume in a simple four-step cycle with a linear isotherm, and showed that large void volume could significantly reduce recovery. Unfortunately, the study applies to an idealized situation without consideration of mass transfer resistance, heat effect and isotherm non-linearity. In addition, only production end void volume and a simple cycle are investigated. U.S. Pat. No. 5,968,233 to Rouge, et al. discloses a similar conclusion of void effects for a more realistic air separation cycle. However, the patent focuses only on production end void volume. More importantly, the patent looks for an optimum void volume for oxygen cost, rather than a minimum void volume.
The effect of void volume varies depending on the location of the void (relative to the adsorbent) and on which step of the cycle the void is considered. For example, the light product gas in the production end void space serves, to some extent, as a purge gas during a countercurrent regeneration step; however, this purge is less effective than a controlled purge at low pressure. Also, the gas and compression work in both the production and feed end voids can be partially recovered during an equalization step. However, the losses of compressed gas and work during regeneration can be significant. In general, lower void volume ratio results in improved process performance (i.e., improved recovery, power consumption, etc. without considering flow distribution requirements and increasing fabrication costs).
Conventional methods for reducing PSA void spaces and improving process performance typically falls into the following classes: (a) vessel system with improved headers, (b) single-vessel system, and (c) piston-driven PSA system.
Vessel system design is a classic approach, focusing only on the vessel or bed itself. Vessels are designed to improve flow distribution and minimize void spaces within the vessel ends (or headers) between the gas inlet (feed end) or outlet (production end) and the adsorbent. U.S. Pat. No. 5,759,242 to Smolarek, et al., which describes a radial adsorber, and U.S. Pat. No. 5,538,544 to Nowobilski, et al., which describes a conventional vertical adsorber, are two recent examples of this approach. However, the vessel design approach has its limits for reducing void space. With a given flow distributor, it can only decrease header space to a level where flow maldistribution and pressure drop are acceptable for the process. In addition, void spaces in distribution pipes are not addressed by this approach.
The single-vessel pressure swing adsorption system disclosed in U.S. Pat. No. 4,194,892 to Jones, et al., is a rapid pressure swing adsorption (RPSA) system, which utilizes a single vessel filled with small adsorbent particles as well as very short cycle times (as short as a few seconds). It is worth noting that U.S. Pat. No. 4,194,892's primary purpose is not to reduce void space, but instead to reduce cycle time and adsorbent inventory and to eliminate multiple vessels. However, the use of a single bed can theoretically eliminate the distribution pipes (and then their voids) connecting a compressor or vacuum pump to different vessels in a multiple vessel PSA. The disadvantages of the single bed PSA are: (1) void space in the vessel header is not addressed and (2) product recovery is limited (in comparison to multiple vessel PSA) without an additional storage tank and recycle component.
U.S. Pat. No. 4,354,859 to Keller, et al. teaches a pressure swing parametric pumping process. This system, called a piston-driven PSA system, uses two synchronized pistons to pressurize and depressurize a single adsorber that is fed near the center of the adsorbent bed. Farooq, et al., Separation and Purification Technology 13 (1998) 181, applied this approach to a parallel passage adsorber for nitrogen and carbon dioxide separation. Suzuki, et al., Adsorption 2 (1996) 111 also utilized this approach in a single piston system for oxygen enrichment from air. This approach can theoretically eliminate header void spaces associated with conventional PSA systems. Moreover, there is no need for valves because flows can be controlled by the piston movement. However, the piston-driven PSA system suffers from scale-up difficulties. For instance, a very large piston is normally required for a large production unit. A piston of this size would be difficult to manufacture. Also, like the single bed PSA, the piston driven PSA suffers from low recovery in comparison to a conventional multiple vessel PSA.
Illustratively, European Patent Application 0,879,630 A2 to Garrett and La Cava extends the piston-driven PSA concept to a system with two sets of pistons or diaphragms. A first set of pistons or diaphragms is located at the top and bottom of the bed to influence fluid flow, as well as a second set of pistons or diaphragms at the side walls of the adsorber. The second set fractionally increases and decreases bed pressure prior to the adsorption and desorption steps, respectively. The Garrett and La Cava patent application teaches that using two sets is more efficient than only using one set. Nonetheless, as acknowledged by Garrett and LaCava, its recovery is low. In addition, the second set of pistons or diaphragms complicates the process.
The present invention addresses the limitations of conventional vessel designs, single vessel PSA, and piston-driven PSA systems. The present invention not only minimizes the header void spaces within each vessel, but also improves distribution pipe void spaces between the compressor/vacuum pump and each vessel. Moreover, the present invention can use multiple compressors and/or vacuum pumps to better distribute flow and further reduce the void spaces.